Geographical isolation is an important constituent of most scenarios of the evolutionary origin of species. In an earlier study, Anthony (1937) compared mammals of Shiva Temple with those of the North Rim of the Grand Canyon, and concluded that isolation on Shiva was incomplete since the two populations demonstrated no significant differences. We attempt here to assess the degree of biological isolation between Shiva Temple and the nearby North Rim of the Grand Canyon. Based on limited direct observation of selected environmental variables, analysis of vegetation, and study of the known distribution of small mammals, we conclude that for some small mammals, Shiva Temple is probably biologically isolated from the North Rim. The implications of these findings are discussed in terms of the creationist concept of a recent and catastrophic origin of the Grand Canyon. Key Words: Grand Canyon, Shiva Temple, geographical isolation, biogeography, speciation, Anthony.